Material processing apparatus, such as plasma arc torches and lasers are widely used in the cutting and marking of metallic materials known as workpieces. A plasma arc torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. Gases used in the torch can be non-reactive (e.g., argon or nitrogen), or reactive (e.g., oxygen or air). The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum.
One method for producing a plasma arc in a plasma arc torch is the contact start method. The contact start method involves establishing physical contact and electrical communication between the electrode and the nozzle to create a current path between them. The electrode and the nozzle can cooperate to create a plasma chamber within the torch body. An electrical current is provided to the electrode and the nozzle, and a gas is introduced to the plasma chamber. Gas pressure builds up until the pressure is sufficient to separate the electrode and the nozzle. The separation causes an arc to be formed between the electrode and the nozzle in the plasma chamber. The arc ionizes the introduced gas to produce a plasma jet that can be transferred to the workpiece for material processing. In some applications, the power supply is adapted to provide a first electrical current known as a pilot current during generation of the arc and a second current known as a transferred arc current when the plasma jet has been transferred to the workpiece.
Various configurations are possible for generating the arc. For example, the electrode can move within the torch body away from the stationary nozzle. Such a configuration is referred to as the “blow-back” contact start method because the gas pressure causes the electrode to move away from the workpiece. In another configuration, the nozzle can move away from the relatively stationary electrode. Such a configuration is referred to as the “blow-forward” contact start method because the gas pressure causes the nozzle to move toward the workpiece. In still another configuration, other torch components (e.g., the swirl ring) can be moved between the stationary electrode and nozzle.
Certain components of the material processing apparatus deteriorate over time from use. These “consumable” components include, in the case of a plasma arc torch, the electrode, swirl ring, nozzle, and shield. Furthermore, in the process of starting the torch using the contact start method, various consumable components can become misaligned, which reduces the useful life of the components as well as the accuracy and repeatability of plasma jet location. Ideally, these components are easily replaceable in the field. Nevertheless, replacing consumable components can result in down time and reduced productivity.
In the blow-back method of contact starting a plasma arc torch, the electrode is moved away from the nozzle to initiate a pilot arc between the electrode and the nozzle. A proximal end of the electrode (e.g., remote from the workpiece) engages a power contact that forms a part of the torch body. Movement of the electrode away from the nozzle also moves the power contact. Repeated use of the torch results in wear on both the power contact and on the electrode. Replacing the electrode is routine in plasma arc torch operation and the process is routinely performed. However, replacing the power contact involves disassembling the torch body and can be time-consuming and expensive because the power contact is not designed to be a consumable component. Some blow-back torches involve moving the power contact with respect to the relatively stationary torch body. Movement of such a power contact and the effectiveness of the torch can be affected by the stiffness or rigidity of the power cable that connects the power contact to the power supply.
For example, FIG. 1 is a cross section of a known contact start plasma arc torch. The system 100 includes a power supply (not shown) in electrical communication over a current-carrying cable 104 with a power contact 108 that provides current to the torch 112. The torch 112 includes a cathode block 116 electrically insulated from and surrounding the power contact 108. The power contact 108 abuts a proximal end 120 of an electrically conductive electrode 124. A spring 128 disposed within the cathode block 116 reacts against a surface 132 of the cathode block 116 to urge the power contact 108 and electrode 124 toward an electrically conductive nozzle 136. The electrode 124 is urged into contact with the nozzle 136 by the spring prior to initiation of an arc for processing a workpiece (not shown).
A current path is established from the cable 104 to the power contact 108, the electrode 124, and the nozzle 136. Electrical current can be passed along the current path. The electrode 124 cooperates with the nozzle 136 to form a portion of a plasma chamber 140. A plasma gas can be supplied to the plasma chamber 140 to increase pressure within the plasma chamber 140 and overcome the force provided by the spring 128. The pressure forces the electrode 124 and the power contact 108 away from the nozzle 136. A potential difference develops between the electrode 124 (e.g., the cathode) and the nozzle 136 (e.g., the anode) as the gap 144 between the electrode 124 and the nozzle 136 increases. An arc (not shown) ionizes gas particles and is initiated across the gap 144 for workpiece processing.
One drawback of the system 100 is that the power contact 108 is required to move as the electrode 124 moves to initiate an arc. As the current carrying capacity of the cable 104 increases, the size of the cable 104 increases, but the flexibility of the cable 104 decreases. The decreased flexibility of the cable 104 reduces the versatility and maneuverability of the torch 112. Additionally, the power contact 108 and the cathode block 116 require relatively tight tolerances (e.g., with relatively small clearance between the power contact 108 and the cathode block 116). The relatively tight tolerances position and guide the power contact 108 during motion of the power contact 108, for example, during initiation of a pilot arc.